How do you like the WT-550? A friend toured nationally with one for years. I thought it sounded great.
I owned both a Shuttlemax 12.0 and a Magellan 800, though not at the same time, and still own the big Genz GBE 1200, which I had concurrently with both - I didn't get to compare the Max and the Magellan directly to each other, but I got to compare each with the GBE, which has been my reference amp for a while. I'm familiar with the other Shuttle heads and the Streamliner to varying degrees.
I can't say what you'll like, but the Max 12.0 and the Magellan are different enough that I think most people will have a preference. The EQ on the 'Max (particularly with the channel blending) is very flexible, but, for me, "fat and round, but clean and articulate" was easier to dial-in quickly with the Magellan.
The Magellan is voiced a little warmer than the Shuttlemax with knobs at noon and it's a little more subdued in the highest frequencies, but it's still pretty flexible and it's not hard to dial in a snappy sound with it (I got a nice, meaty slap tone with it and my Brubaker Brute). In general, I liked the touch on it better than I did that of the Shuttlemax -- I felt more connected to my instrument with it and felt it hit harder -- and I think it's a punchier amp overall (though neither amp hits as hard as the GBE, particularly in the lowest part of the register).
The Magellan will have more headroom into a single cab (4 or 8 Ohm) than the Max 12.0 will. The Max, as you know, will run two 4 Ohm cabs, while the Magellan won't (2.7 Ohms minimum).
The Max will do some things that the Magellan won't and I can see preferring it in some situations, but if I had to play a gig with one tomorrow, particularly given the music and the tonal preferences you mention, I'd take the Magellan.
You may end up disagreeing, but that's my take.